The invention relates to a method for translating or “transcoding” between formats of compressed digital signals that have been encoded using a discrete cosine transform (DCT) operation. More particularly, the invention relates to a method of transcoding between a DV compressed signal and an MPEG compressed signal without performing an inverse discrete cosine transform (IDCT) operation.
The Moving Picture Experts Group (MPEG) video compression standard is one of the most popular formats in digital video production for compressing or encoding a video signal. The MPEG-2 standard was initially proposed and developed as an open standard by the international standard organization (ISO), its main strength is its flexibility and compression efficiency. The MPEG-2 standard is well suited for data transmission and storage. This standard was designed to allow video signals in many different formats to be compressed. It was also designed for broadcasting applications with a point to multi-point model in mind. As such, the MPEG-2 encoding-decoding process is highly asymmetric, the MPEG-2 encoder, for example, being far more complex than its corresponding decoder.
The MPEG-2 video compression method is also lossy, that is to say, significant information is lost when the signal is compressed. The standard is designed, however, to take advantage of the human visual system in order to hide these losses. Nonetheless, significant degradation of the video image occurs if a compressed signal is expanded, re-compressed and expanded again.
Consequently, MPEG based compression is not well suited for production and/or editing applications. In a production/editing environment there would be as many encoding steps as decoding steps, and the processing involved for simple tasks (e.g. frame-level editing) would be unwieldy as a complex encoding process would be required after each edit.
Because of these features of MPEG, a second standard Digital Video(DV) was developed as a proposed SMPTE standard. This standard includes Digital Video at 50 Mb/s (DV50) and Digital Video at 25 Mb/s (DV25). The DV standard is typically utilized to compress data for production/editing applications. The compression methods defined by the DV standard were designed with a studio production scenario in mind, so encoder and decoder are of roughly similar complexity. The DV standard uses fixed size frames and does not place as much emphasis on compression efficiency as does MPEG-2.
More detailed features of the DV standard are briefly reviewed hereafter for the purpose of facilitating the teachings of the present invention.